liguy, there is no theoretical reason why a properly set-up 2-channel subwoofer system would not be in phase - unless one subs' speaker leads were hooked up backwards from the other - just like w/a pair of regular full-range speakers, being driven by a stereo amp. of course, you could also have your subs out-of-phase from your monitors, if the subs' leads were reversed relative to the monitors', but this mix-up is also possible w/a single sub. in fact, assuming proper connections, there's actually a chance of signal-cancellation when using only *one* sub, w/the two-channel signal summed to mono. in this set-up, there's the possibility of left/right-channel signal cancellation...
regards, doug s.